An injection moulding tool of this type is known e.g. from U.S. Pat. No. 5,533,882. In this device, a nozzle holder rests directly on the hot-runner manifold plate, a distributor channel being directly connected to a supply channel in the nozzle holder. The nozzle is arranged at the nozzle holder end facing away from the hot-runner manifold plate. The supply channel merges at this point with the nozzle opening, said nozzle opening being adapted to be opened and closed by a valve stem. The valve stem is supported in the nozzle holder such that it is axially displaceable therein. The movement of the valve stem is produced by a sleevelike piston which is coaxially arranged around the nozzle holder. The piston chamber is defined by a sleeve surrounding the nozzle holder and by a closure plate. In order to establish a connection between the valve stem and the piston, a comparatively complicated yoke structure has to be used for connecting said needle and said piston through a lateral opening. It is true that this kind of structural design already provides a valve-stem closure means on the side of the cavity, but this closure means is comparatively complicated as far as its structural design is concerned so that improvements are desirable.
In the case of nozzle holders resting on the hot-runner manifold plate, needle closure means including a shut-off needle which extends through the hot-runner manifold plate have normally always been used up to now. The piston used for controlling the shut-off needle was then provided in a separate means at the back of the hot-runner manifold plate. However, such a structural design always necessitated a comparatively long shut-off needle, and this rendered the structural design comparatively complicated and expensive. A structural design of this type is known e.g. from EP 0647514A1.
In the case of another structural design, the hot-runner manifold plate has been subdivided into a main hot-runner manifold plate and into cooler individual plates. Although the piston units could-there be accommodated in the cooler individual plates, a connection channel which could be heated less strongly was necessary between the main hot-runner manifold plate and the nozzle holder resting on the individual plate. Also this structural design is comparatively complicated and expensive, since additional components are required. Furthermore, temperature control problems may arise in the case of this kind of embodiment. Such a device is known e.g. from EP0374353A2.